I have had some great results by adding grape tannin a few days before bottling. Just make sure the wine is completely cleared... if you plan on filtering, add the tannin a day or so later, and bottle a day or so after that.
The best results I had was with the WE Petit Verdot where I added almost a full bottle of liquid grape tannin. The wine really took on that dry sensation and was not at all bitter. Since then I have added some liquid grape tannin to most of my kits (at least the kits where it would make most sense to want it). Very nice, but you do need to experiment to get to the point where you like it.
As for adding mushrooms to a wine - I'm interested to hear the results, but my first reaction is to be wary of that. Mushrooms are a fungus, and lots of things could be introduced into that wine. But I suppose the same thing could be said about adding chunks of wood ;-)
I was curious so I did a quick google search and found a reference that the earthy flavour is a result of geosmin which is an organic compound. There was also a reference that this compound is responsible for the earthy flavour of beets... so perhaps some washed baby beets might do the trick also.. start with whole and cut them down to the point where they fit in the carboy neck, and so that you can get them out.
I would also first start off with a grape that has an earthy quality - like a Pinotage, and go from there.